Products made from aluminum alloy casting are widely used as the parts of automobile vehicles, household electric apparatuses and the like. Those cast material are brazed one to another or to other expanded metal parts. The die-casting is suited to manufacture products of a sophisticated shape and having a smooth surface, rapidly and at a higher productivity. However, such die-cast material often suffers from a certain defect known as `blister`, when subjected to considerably raised temperatures. Thus, it has been difficult or impossible to employ material of die-cast aluminum alloy if they have to be heated to 450.degree. C. and above when they are brazed.
The blister appearing on cast material is a partial outward expansion thereof. This problem is an irreversible deformation of surface layer of said cast material, due to thermal expansion of entrained air or hydrogen gas that was entrained into the article when a molten metal was forced into a mold. In other words, the surface layer yields or succumbs to a high internal pressure of the heated air or gas.
A reduced amount of the entrained gas has been expected to diminish the `blistering`. Thus, the so-called slow squeeze-casting and SSF (semi-sintered piece forging) were proposed. By the squeeze-casting method, a molten metal is injected at a decreased rate into a mold, so as to effect the forging of a molten metal. It may also be useful to optimize the injection condition for the ordinary die-casting process in such a manner that the amount of entrained gas is reduced and/or prevented from concentrating in a surface layer of each cast material.
Productivity of squeeze-casting and SSF processes is much poorer than that of ordinary die-casting, and less suited to mass production of cast material. As seeking of optimum conditions takes long, and as a strict and severe control of ordinary casting process is mandatory, the operation efficiency is much lower than the ordinary die-casting. In consequence, it has been difficult for die-cast material to be brazed.